A polarizing film prepared by adsorbing a dichromatic dye, etc., to a stretched film, such as a stretched polyvinyl alcohol film, etc., cannot be effectively used because at least 50% of an incident light is absorbed, and thus it is difficult to increase a luminance and decrease a consumed electric power of a liquid crystal display device, etc. As a result, an optical element having a circular dichroism has been expected. This is because a helical axis of a liquid crystal molecule causes a vertical grandjean orientation to the optical element, in the natural light striking in parallel to the helical axis (incident angle: 0 degree), about a half of the light of a certain wavelength is reflected as a right (or left) circularly polarized light, residual about a half of the light is transmitted as a left (or right) circularly polarized light, the wavelength .lambda. is determined by the equation .lambda.=n.multidot.p (wherein n is a mean refractive index of liquid crystal and p is helical pitch of a cholesteric phase), the right or left reflected circularly polarized light is determined by the helical state of the cholesteric phase and coincides with the circulation direction of the helix, and thus there is a possibility that the reflected light can be utilized in addition to the transmitted light by the separation of the natural light as described above.
Hitherto, as an optical element having a circular dichroism, an optical element formed by enclosing a liquid cholesteric liquid crystal made up of a low molecular weight material between substrates such as glasses in an oriented state and an optical element made up of a liquid crystal polymer showing a cholesteric liquid crystal phase are known as disclosed in Japanese Patent Publication (unexamined) No. 55-21479 and U.S. Pat. No. 5,332,522.
However, in the former low molecular weight material, because substrates are used, the optical element becomes thick and heavy and there is a problem of hindering light-weighting and thinning of the liquid crystal display. Also, there is a problem that the orientation state of the liquid crystal, for example, a pitch thereof is liable to be changed by the change of temperature, etc.
On the other hand, in the case of using the latter liquid crystal polymer, it is difficult to obtain a solid material such as a film of a good oriented state as a low molecular weight material, a long time such as several hours, etc., is required for the orientation treatment, the glass transition temperature thereof is low, or the durability is insufficient, resulting in poor practical use. Thus, in any cases, it is difficult to obtain an optical element having a circular dichroism in a solid state, in particular, the optical element for visible light.
For the purpose of improving the solid state, various attempts of combining monomers, in particular changing monomers having a cholesteric phase-imparting property have been made, but at present the problems of being poor in liquid crystal orienting property and heat resistance have not yet been solved. For example, to obtain a cholesteric type liquid crystal polymer wherein the selected reflected wavelength is in a visible light region, it is necessary that the copolymerization ratio of a monomer having the cholesteric phase-imparting property is at least about 15% by weight, and liquid crystal property and heat resistance of the polymer obtained are greatly decreased.
Accordingly, an object of the present invention is to provide a monomer having a large torsion force defined by 10.sup.4 /[selectively reflected wavelength (nm).times.copolymerization ratio (mol %), to provide a liquid crystal polymer using the monomer, which is excellent in a film-forming property, can easily control a helical pitch of a cholesteric phase, can form a grandjean orientation in a good mono-domain state by an orientation treatment of a short time such as few minutes, and can stably fix it in a glass state, and to provide an optical element having a circular dichroism made up of a solidified product of the liquid crystal polymer, which is thin and light, is hard to change the oriented state such as pitch, etc., at a practical temperature, and has excellent durability and storage stability.